1. Technical Field
The present invention relates generally to ion implanters, and more particularly, to a method and apparatus for confining electrons inside a magnetic region of a magnet of an ion implanter.
2. Related Art
Improving productivity in ion implanters that use a low energy beam is a continuing issue in the ion implanter industry. One area of focus is improving beam transport efficiency. In particular, on single wafer ion beam implant systems, the ion beam is decelerated from high energy to low energy by an electrostatic bi-potential lens positioned before an analyzer magnet. An analyzer magnet functions to refine an ion beam by selecting appropriate ions for the ion beam. In any ion beam, ions of like charge tend to repel each other, which cause the ion beam to disperse. As an ion beam decelerates from high energy to low energy, the ion dispersion problem increases, which may result in the ion beam transport loss due to the repulsion.
As shown in FIG. 1, one mechanism to reduce ion beam 2 dispersion is to maintain a sufficient amount of free electrons 4 (e.g., via beam collisions with neutral atoms 6) within ion beam 2 to neutralize the net space charge of ion beam 2. Free electrons 4 can be introduced from an external source (not shown) to ion beam 2 in the regions where there is no magnetic field. Free electrons 4 can also be produced by ion beam 2 colliding with surfaces (not shown) about it, and from ion beam 2 colliding with residual gas (not shown). However, heating of free electrons 4 from collisions produces hot electrons 4H that can escape from ion beam 2 to the walls of the beam-line. As this occurs, electrons 4L are lost, thereby increasing the net space charge of ion beam 2 when additional free electrons 4 are not supplied to ion beam 2. The space charge neutralization problem is heightened inside a magnetic region 10 of a magnet 12 because free electrons 4 cannot move into magnetic region 10 from outside of magnetic region 10 as they orbit about magnetic field lines 16. The replacement of lost electrons 4L in magnetic region 10 is usually accomplished by relying on ion beam 2 generated electrons. At the same time, the magnetic field of magnet 12 tends to guide hot electrons 4H to migrate to planar poles 14 of magnet 12 and thus enhance the exit of lost electrons 4L from ion beam 2. This situation can lead to ion beam 2 expansion and further cause ion beam 2 transport losses if ion beam 2 generated electrons in magnetic region 10 are not adequate to replace lost electrons 4L. It should be recognized that while magnet 12 is illustrated as an analyzer magnet, the problem is not limited to this particular magnet within an ion implanter.
In view of the foregoing, there is a need in the art for improved beam transport efficiency in a magnet of an ion implanter.